feat(sync): networkHead callback metric

sync/metrics.go

@@ -11,6 +11,9 @@ import (
 var meter = otel.Meter("header/sync")
 
 type metrics struct {
+	networkHead      atomic.Int64
+	networkHeadGauge metric.Int64ObservableGauge
+
 	totalSynced      atomic.Int64
 	totalSyncedGauge metric.Float64ObservableGauge
 }
@@ -23,16 +26,33 @@ func newMetrics() (*metrics, error) {
 	if err != nil {
 		return nil, err
 	}
+	netHead, err := meter.Int64ObservableGauge(
+		"network_head_gauge",
+		metric.WithDescription("network head"),
+	)
+	if err != nil {
+		return nil, err
+	}
 
 	m := &metrics{
 		totalSyncedGauge: totalSynced,
+		networkHeadGauge: netHead,
 	}
 
-	callback := func(ctx context.Context, observer metric.Observer) error {
+	totalSyncedCallback := func(ctx context.Context, observer metric.Observer) error {
 		observer.ObserveFloat64(totalSynced, float64(m.totalSynced.Load()))
 		return nil
 	}
-	_, err = meter.RegisterCallback(callback, totalSynced)
+	_, err = meter.RegisterCallback(totalSyncedCallback, totalSynced)
+	if err != nil {
+		return nil, err
+	}
+
+	netHeadCallback := func(ctx context.Context, observer metric.Observer) error {
+		observer.ObserveInt64(netHead, m.networkHead.Load())
+		return nil
+	}
+	_, err = meter.RegisterCallback(netHeadCallback, netHead)
 	if err != nil {
 		return nil, err
 	}
@@ -48,3 +68,11 @@ func (m *metrics) recordTotalSynced(totalSynced int) {
 
 	m.totalSynced.Add(int64(totalSynced))
 }
+
+func (m *metrics) recordNetworkHead(netHead uint64) {
+	if m == nil {
+		return
+	}
+
+	m.networkHead.Store(int64(netHead))
+}

sync/sync_head.go

@@ -121,11 +121,14 @@ func (s *Syncer[H]) setSubjectiveHead(ctx context.Context, netHead H) {
 			"err", err)
 	}
 
+	defer s.metrics.recordNetworkHead(netHead.Height())
+
 	storeHead, err := s.store.Head(ctx)
 	if err == nil && storeHead.Height() >= netHead.Height() {
 		// we already synced it up - do nothing
 		return
 	}
+
 	// and if valid, set it as new subjective head
 	s.pending.Add(netHead)
 	s.wantSync()